Eileen,
I know that I am dense, but I am not sure I understand your instructions.
The way I interpret the directions -
1. Subtract seam allowance from original pattern piece
2. Multiply the pattern piece minus the seam allowance by the % that you want to make the entire pattern smaller.
3. Add the seam allowance back onto the pattern piece.
Also, I don't know what you meant by taking into account that Supernova is made in two halves when you do the math. Can you tell that I am not a math professor? LOL
Thanks in advance, Judy